1. Field of the Invention
The present invention relates to single pass plate and frame heat exchangers, and more specifically to an apparatus and method for chemically cleaning a single pass plate and frame heat exchanger without disassembly, with the use of portable, lightweight equipment.
2. Description of Related Art
Heat exchangers have been used to provide for domestic hot water, and in commercial, industrial, process, marine, aquaculture, HVAC, heat recovery, refrigeration, food and beverage processing, pharmaceutical, clean room, and many other applications for many years. One of the most common types of heat exchangers is the plate and frame heat exchanger. A plate and frame heat exchanger is generally comprised of a gasketed plate and frame construction. Gasketed plate and frame heat exchangers have a series of channeled plates that are mounted in a frame and clamped together. Each plate is commonly made from pressable materials, such as stainless steel, hastalloy, titanium, and the like. Each plate is further formed with corrugations and peripherally surrounded by an elastomeric gasket. The plate corrugations provide support between plates and encourage turbulence, which gives the plate and frame heat exchanger its high heat transfer. The gaskets are made of a suitable material generally for a specified thermal duty, and placed between the plates to contain the pressure, seal the fluid, and direct the flow. In a brazed-type plate heat exchanger, the gaskets are replaced with a brazed material, preferably copper or nickel, which allows for an increase in the operational pressure and temperature capabilities of the heat exchanger. The gasketed plates are assembled in a bundle or pack, mounted on upper and lower guide rails, and compressed between two end frames by compression bolts. The frame provides structural support and pressure containment by tightly enclosing the plate pack. The mainframe components generally consist of a fixed end, a movable end, upper carrying bar and lower guide bar, and tightening bolts. The end frames and heavy threaded rods compress the plate pack to create a highly efficient heat transfer device. The fluid flows through ports located on the frames. The location of these ports is generally dependent upon the particular flow characteristics required by the heat transfer application. The most common plate heat exchanger type is where the fluids pass through the heat exchanger in one direction only. This is referred to as a single pass unit, and in this configuration all of the inlet and outlet ports are located on the fixed end plate. FIG. 1 depicts a single pass plate heat exchanger 10 with arrow indications for the directional flow for the higher temperature fluid 12 and the lower temperature fluid 14. Fluid to fluid heat exchange is accomplished through both conductive and convective heat transfers. Conductive heat transfer is primarily a function of the heat transfer surface area. With plate and frame heat exchangers, increasing the surface area means adding plates. Convective heat transfer is a function of turbulence; the more turbulence, the more heat transfer.
As fluids pass through a plate and frame heat exchanger, certain fouling deposits will accumulate on the plate surfaces. This reduces the thermal efficiency of the heat exchanger. Dirt, microbiological material, iron oxide deposits, scale, and process contaminants will negatively impact almost any piping system if left to accumulate, including plate and frame heat exchangers. As deposits accumulate on the plate and frame heat exchanger plates, heat transfer efficiency is reduced and operating costs are increased. In most cases, the plate and frame heat exchanger is disassembled to pressure wash the plates. This procedure is time consuming and can easily take several days to complete when the heat exchanger contains several hundred plates. The pressure wash will remove many fouling deposits completely, but there are also many types of deposits that cannot be easily washed away. For certain types of hard deposits, the plates are normally removed from the frame and cleaned at a facility where they can be individually dipped in a chemical solution designed to dissolve the deposits. Moreover, other drawbacks to physical pressure washing may be realized. For example, the gaskets may separate from the plate surface and will need realignment before the plate and frame heat exchanger can be securely closed. Thus, after pressure washing, the gaskets and the adjacent sealing surfaces have to be inspected carefully to ensure that no debris is present. Debris on the gasket surface, even of a small nature, can cause the heat exchanger to leak when it is subsequently assembled and ready for operation. In this event, the heat exchanger would necessarily have to be reopened, rinsed, and re-closed. Furthermore, when the heat exchanger is opened, it has to be isolated from the system piping by closing the inlet and outlet valves for both fluids. Both fluids must then be drained, even if both sides of the heat exchanger are not fouled. In some cases, it would be beneficial not to drain the clean side fluid because it may be a toxic or hazardous material that must be contained. Furthermore, a brazed-type plate heat exchanger cannot be taken apart and cleaned in this manner.
Given the significant limitations of pressure washing individual plates, manufacturers have opted for a continuing, multistage chemical cleaning program using aggressive chemical agents. Chemical cleaning usually requires a period of exposure within the system to dissolve and flush deposits, and then to neutralize the system. Plate and frame heat exchanger manufacturers generally recommend chemically cleaning without opening the heat exchanger. As typically recommended by a plate and frame heat exchanger manufacturer, the cleaning solution is pumped into an existing heat exchanger inlet and returned to a circulation tank from an existing heat exchanger outlet. Problematically, unless a large high-pressure pump is used to force the chemical solution through the plate and frame heat exchanger, there is generally an insufficient flow rate that prevents the cleaning solution from reaching the opposite end of the heat exchanger. In effect, using even the most powerful portable pumps available in the industry today, the pumped cleaning solution is “short circuited” by the mass and inertia of the existing fluid downstream within the heat exchanger that the cleaning solution must ultimately move and replace. FIG. 2 depicts a plate frame heat exchanger 20 with cleaning solution applied under pressure at inlet point 22. Flow circuit 24 is the desired result, having the cleaning solution traverse to the opposite end of the heat exchanger and up through the plates, and return at outlet point 26. However, flow circuit 28 represents the more common situation of cleaning solution flow, where the input cleaning solution, under portable pump pressure does not have, nor can it deliver, enough force or pressure to move through to the end of the heat exchanger, which leaves the plates at the end of the plate pack with little or no exposure to the cleaning solution. Since flow circuit 28 represents the path of least resistance for the cleaning solution under portable pump pressure and flow rate, flow circuit 24 is never realized. To achieve enough flow to allow proper distribution across all of the plates would require a pump that was almost as large as the system pump, and a large circulation tank with large hose attachments. This would also require a large footprint in the mechanical room, and would not be portable to any other heat exchanger elsewhere in the facility. Using a smaller portable system with its standard pump, a uniform distribution flow of cleaning solution throughout each heat exchanger plate cannot be realized. Nor can debris of any significant size be readily removed or vacuumed under the standard portable configuration.
Some prior art techniques for introducing fluids or gases in plate heat exchangers have been attempted, although none have been introduced for the large single pass plate and frame heat exchangers. For example, in U.S. Pat. No. 4,562,885 issued to Pausch on Jan. 7, 1986 entitled, “PLATE HEAT EXCHANGER AND PRESSURE BLAST CLEANER,” a plate heat exchanger is disclosed having a plurality of parallel channels for conveying heated gases. A pressure tank is positioned adjacent the housing, and is in flow communication with the interior of the housing via a plurality of jet pipes. Each of the jet pipes has a closed end and a plurality of air jet passages or nozzles formed along one surface. Each jet pipe center is positioned between a pair of open-ended plates. Jet nozzles face toward the open-ended area. Although Pausch teaches a number of pipes having a plurality of passages or nozzles for cleaning a plate heat exchanger, the apparatus requires many pipes, one for each opening between the plates, which could not be realistically utilized in large plate and frame heat exchangers without significant redesign. Furthermore, in Pausch, the air jet passages or nozzles within each pipe are designated along the opening between two plates, and not transverse to this direction. By Pausch's design, a pipe is needed for every space or opening between plates, and the plurality of nozzles for each pipe are dedicated to one opening. Furthermore, Pausch's invention is designed to administer multiple air blasts resulting in a sonic wave of pressurized air forming downstream of the jet pipe, and passing through the airflow channel with which the jet pipe is aligned. It is not designed for reverse extraction of applied cleaning fluid or debris.
In European Patent No. EP 0877222A2 issued to Sabin on Nov. 11, 1998, entitled “DEVICE FOR INJECTING PRESSURIZED FLUIDS IN A PLATE-LIKE HEAT EXCHANGER AND PROCESS FOR CLEANING THIS INJECTION DEVICE,” an injection tube is disclosed having calibrated orifices, and extending over a plate bundle. The injection tube has a closed end and an open end connected to a supply pipe. The open end is connected to a pipe for evacuating particles deposited on a filter placed inside the tube. Two identical tubes are used. One is shown towards the top of the heat exchanger, while the other is situated along the bottom of the plates. As shown, Sabin does not teach or disclose using this apparatus for a plate and frame heat exchanger. Nor does Sabin teach of positioning the apertures of one cleaning tube directionally with respect to the other tube's apertures to facilitate particulate removal. In fact, if a reverse cleaning solution flow were attempted using the Sabin design, Sabin's filters would prevent particulates within the plate heat exchanger from exiting the tubes. Consequently, the Sabin design cannot accomplish reversing the cleaning fluid flow and extracting particulates from a plate and frame heat exchanger.
U.S. Pat. No. 4,666,531 issued to Minard on May 19, 1987 entitled, “DEVICE AND METHOD FOR CLEANING FIN-TYPE HEAT EXCHANGERS IN AIR DUCTS,” discloses a cleaning system having a pair of tubes mounted horizontally across an air duct. Plastic tubes are mounted on the downstream and upstream sides, respectively, of the heat exchanger. The tubes also run parallel to the edges of the fins. Tubes are shown having a plurality of slots extending longitudinally along the tube outer diameter in two closely spaced rows. The slots are cut to produce a fan-shaped spray, as liquid from tube sprays through the slots. In Minard, both tubes are located at the top of the air duct, and used for cleaning and rinsing fluid ingress. Neither tube is situated or utilized for fluid extraction. Furthermore, the cleaning fluid travels from alternate tubes at the top, and traverses down the plates under the force of gravity. Minard also does not teach or disclose any particular positioning of the slotted tubes such that debris is less likely to clog them and restrict fluid extraction. Nor does Minard does teach or disclose the aperture placement on the tubes for fluid egress.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a portable apparatus and method for cleaning plate and frame heat exchangers that allows for substantially uniform disbursement of a chemical cleaning solution to each of the plates in the heat exchanger.
It is another object of the present invention to provide a portable apparatus and method for cleaning plate and frame heat exchangers that extracts and removes measurable particulates from the heat exchanger during cleaning.
A further object of the invention is to provide a portable apparatus and method for cleaning plate and frame heat exchangers that provides for reverse extraction of the applied cleaning fluid.
It is yet another object of the present invention to provide a portable apparatus and method for cleaning plate and frame heat exchangers that diminishes the amount of clogging due to debris, and thus facilitates fluid extraction during cleaning.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.